HELP: Possible Glitch???

In the early stages of the game when you have to kill 100 creatures in the sewers of Baldur's Gate I ran into a problem.....I can only find 99 of them and I scoured every nook and cranny of the sewers. What gives???

A lot of people have complained about this, and often the problem is just missing a rat, not because of a bug. When the rats are in the water, they're difficult to see, and there's one area in particular where you can run in circles and possibly be running in the same direction as the rat (making it harder to find it).